616 DOC. 594 AUGUST 1918 594. To Friedrich Adler [Ahrenshoop,] 4 August 1918 Dear Adler, Yesterday your manuscript arrived.[1] I have already studied the first chapter and am thus informed about all the essentials.[2] The basic physical assumptions are:[3] a) no Lorentz deformation of moving rigid bodies, b) no influence of motion on the running rate of the clocks. Thus the geometrical and kinematic elements are entirely given, quite apart from the mathematical expression in coordinates. The assumptions (a) and (b) are, in principle, directly verifiable, but not in practice. (a) leads immediately to Abraham’s theory of motion of an electron, however, which is refuted empirically as soon as the nonexistence of the deforma- tion of moving bodies is applied also to the electron.[4] It must be considered, furthermore, that (a) leads straight away to a contra- diction with the outcome of the Michelson experiment, if it is assumed that the law of the constancy of the velocity of light in a vacuum, which you are not likely to question, is valid relative to the preferred reference system K. I find that a theory can be taken seriously from the physical standpoint only when it does justice to the following observational results: 1) Fizeau’s experiment. 2) Motion of electrons in an electromagn. field. 3) Aberration law. 4) Michelson’s experiment. For it was these facts which compelled the formulation of the special theory of relativity. You have made no attempt to address these fundamental facts, however.- I come now to the formal aspect. Basically, on making arbitrary assumptions about the behavior of measuring rods and clocks, one can use arbitrary transfor- mation equations without coming into conflict with the logic or with experience. If, however, as corresponds with your assumptions about the behavior of the measuring rods and clocks, it is assumed that rigid bodies at rest relative to one another follow Euclidean geometry regarding the positioning laws [Lagerungsge- setze], and that static clocks, relative to one another, run equally quickly, then it is certainly appropriate (although logically absolutely not necessary) to choose the coordinates in such a way that for all legitimate systems the following is valid: